1. Field of the Invention
The present invention relates, generally, to a method and apparatus which protects lean burn catalytic-based emissions control systems. Lean burn emissions systems as used herein broadly includes catalysts and associated equipment which is generally located in the effluent stream of a lean burn combustion system, e.g. in the exhaust or the like. Lean burn combustion systems are combustion systems that operate under lean conditions by design for some portion of the operating time. Examples of lean burn combustion systems include diesel engines, heterogeneously charged direct injection gasoline engines, and other systems that operate under lean conditions for a significant amount of operating time. The exhaust treatment system for these applications operate to reduce exhausts emissions under lean conditions and may well operate under other conditions. The invention contemplates the addition of various compounds to a fuel, for example a low sulfur fuel, or lubricant to protect the emissions systems from poisoning by exhaust byproducts, as well as methods of improving the performance of emissions hardware by protecting emissions systems from poisoning from impurities found in fuel and lubricant sources and increasing the catalyst durability in these systems.
More specifically, the present invention relates to fuel or lubricant compositions containing an organometallic compound which acts as a scavenger to reduce the impact of poisons such as sulfur, phosphorus or lead on catalytic emissions control systems (e.g., a catalyst or trap) used for reducing tailpipe emissions, thereby contributing to lowered emissions characteristics and improved emissions system efficiency and improved emission hardware (e.g., catalyst) durability.
2. Description of the Prior Art
It is well known in the automobile industry to reduce tailpipe emissions by using various strategies. The most common method for reducing emissions from spark ignition engines is by careful control of the air-fuel ratio and ignition timing. For example, retarding ignition timing from the best efficiency setting reduces HC and NOx emissions, while excessive retard of ignition increases the output of CO and HC. Increasing engine speed reduces HC emissions, but NOx emissions increase with load. Increasing coolant temperature tends to reduce HC emissions, but this results in an increase in NOx emissions.
It is also known that treating the effluent stream from a combustion process by exhaust after treatment can lower emissions. The effluent contains a wide variety of chemical species and compounds, some of which may be converted by a catalyst into other compounds or species. For example, it is known to provide exhaust after treatment using a three-way catalyst and a lean NOx trap. Other catalytic and non-catalytic methods are also known.
Thermal reactors are noncatalytic devices which rely on homogeneous bulk gas reactions to oxidize CO and HC. However, in thermal reactors, NOx is largely unaffected. Reactions are enhanced by increasing exhaust temperature (e.g. by a reduced compression ratio or retarded timing) or by increasing exhaust combustibles (rich mixtures). Typically, temperatures of 1500xc2x0 F. (800xc2x0 C.) or more are required for peak efficiency. Usually, the engine is run rich to give 1 percent CO and air is injected into the exhaust. Thermal reactors are seldom used, as the required setting dramatically reduces fuel efficiency.
Catalytic systems are capable of reducing NOx as well as oxidizing CO and HC. However, a reducing environment for NOx treatment is required which necessitates a richer than chemically correct engine air-fuel ratio. A two-bed converter may be used in which air is injected into the second stage to oxidize CO and HC. While efficient, this procedure results in lower fuel economy.
Single stage, three way catalysts (TWC""s) are widely used, but they require extremely precise fuel control to be effective. Only in the close proximity of the stoichiometric ratio is the efficiency high for all three pollutants, excursions to either side of stoichiometric can cause increases in hydrocarbon and carbon monoxide or NOx emissions. Such TWC systems can employ, for example, either a zirconia or titanium oxide exhaust oxygen sensor or other type of exhaust sensor and a feedback electronic controls system to maintain the required air-fuel ratio near stoichiometric.
Catalyst support beds may be pellet or honeycomb (e.g. monolithic). Suitable reducing materials include ruthenium and rhodium, while oxidizing materials include cerium, platinum and palladium.
Diesel systems raise a different set of challenges for emissions control. Strategies for reducing particulate and HC include optimizing fuel injection and air motion, effective fuel atomization at varying loads, control of timing of fuel injection, minimization of parasitic losses in combustion chambers, low sac volume or valve cover orifice nozzles for direct injection, reducing lubrication oil contributions, and rapid engine warm-up.
In terms of after treatment, it is known that diesel engines generally burn lean and the exhaust will therefore usually contain excess oxygen. Thus, NOx reduction with conventional three-way catalysts is not feasible. NOx is removed from diesel exhaust by either selective catalytic reduction, the use of lean NOx catalysts such as those comprised of zeolitic catalysts or using metals such as iridium, or catalyzed thermal decomposition of NO into O2 and N2.
Diesel particulate traps have been developed which employ ceramic or metal filters. Thermal and catalytic regeneration can burn out the material stored. Particulate standards of 0.2 g/mile may necessitate such traps. Both fuel sulfur and aromatic content contribute to particulate emissions. Catalysts have been developed for diesels that are very effective in oxidizing the organic portion of the particulate.
Improved fuel economy can be obtained by using a lean-burn gasoline engine, for example, a direct injection gasoline engine, however currently NOx cannot be reduced effectively from oxidizing exhaust using a typical three-way catalyst because the high levels of oxygen suppress the necessary reducing reactions. Without a NOx adsorber or lean NOx trap (LNT), the superior fuel economy of the lean-burn gasoline engine cannot be exploited. The function of the LNT is to scavenge the NOx from the exhaust, retaining it for reduction at some later time. Periodically, the LNT must be regenerated by reducing the NOx. This can be accomplished by operating the engine under rich air-fuel ratios for the purpose of purging the trap. This change in operating conditions can adversely effect fuel economy as well as driveability. These LNT""s may also be placed on diesel engines, which also operate in a lean air-fuel mode. As in the lean-burn gasoline engines, the exhaust of both types of engines is net oxidizing and therefore is not conducive to the reducing reactions necessary to remove NOx. It is an object of the present invention to improve the storage efficiency and durability of the LNT and to prolong the useful life of the LNT before regeneration is necessary.
It is well known that NOx adsorbers are highly vulnerable to deactivation by sulfur (see, for example, M. Guyon et al., Impact of Sulfur on NOx Trap Catalyst Activity-Study of the Regeneration Conditions, SAE Paper No. 982607 (1998); and P. Eastwood, Critical Topics in Exhaust Gas Aftertreatment, Research Studies Press Ltd. (2000) pp.215-218.) and other products resulting from fuel combustion and normal lubricant consumption. It is an object of the present invention to provide fuel or lubricant compositions capable of reducing the adverse impact of sulfur, and other exhaust byproducts, on the emissions system including NOx adsorbers and LNTs.
Performance fuels for varied applications and engine requirements are known for controlling combustion chamber and intake valve deposits, cleaning port fuel injectors and carburetors, protecting against wear and oxidation, improving lubricity and emissions performance, and ensuring storage stability and cold weather flow. Fuel detergents, dispersants, corrosion inhibitors, stabilizers, oxidation preventers, and performance additives are known to increase desirable properties of fuels.
Organometallic manganese compounds, for example methylcyclopentadienyl manganese tricarbonyl (MMT), available from Ethyl Corporation of Richmond, Va., is known for use in gasoline as an antiknock agent (see, e.g. U.S. Pat. No. 2,818,417). These manganese compounds have been used to lower deposit formation in fuel induction systems (U.S. Pat. Nos. 5,551,957 and 5,679,116), sparkplugs (U.S. Pat. No. 4,674,447) and in exhaust systems (U.S. Pat. Nos. 4,175,927, 4,266,946, 4,317,657, and 4,390,345). Organometallic iron compounds, such as ferrocene, are known as well for octane enhancement (U.S. Pat. No. 4,139,349).
Organometallics for example compounds of Ce, Pt, Mn or Fe among others have been added to fuel to enhance the ability of particulate traps to regenerate or to directly reduce the emissions of particulate from diesel or compression ignition type engines or other combustion systems. These additives function through the action of the metal particles that are the product of additive breakdown on the particulate matter during combustion or in the exhaust or particulate trap.
The present invention contemplates supplying, in a spark- or compression ignition lean combustion system a sufficient amount of an organometallic compound to effectively reduce the impact of poisoning substances on catalytic-based emissions control systems for fuel-combustion systems.
The combustion of a fuel containing an organometallic manganese compound, such as MMT, results in mixtures of manganese compounds containing, among others, species of manganese oxides, manganese phosphates and manganese sulfates. As used hereinafter, a stoichiometric ratio will be referred to using lambda, which is calculated using the following formula:   lambda  =                              air          /          fuel                ⁢                  xe2x80x83                ⁢        ratio                    stoichiometric        ⁢                  xe2x80x83                ⁢                  air          /          fuel                ⁢                  xe2x80x83                ⁢        ratio              .  
When lambda=1, the system is stoichiometric. When lambda greater than 1 the system is a lean system. When lambda less than 1, the system is a rich system. The lean burn combustion systems of the present invention are combustion systems that operate lean by design for a majority of the operating time. Lean combustion systems of the present invention are those systems operating at a lambda of  greater than 1.2, preferably  greater than 1.4 and more preferably  greater than 1.5.
In a gasoline or diesel engine that is operating with excess air according to the present invention, under lean conditions, and using a fuel containing an organometallic compound according to the present invention, the metal will combine with, e.g., sulfur to form, e.g., metal sulfates in the exhaust. These compounds are not stable at the high temperatures typically found in the exhaust manifold or those associated around typical three way catalysts. However, at lower temperatures under which lean NOx catalysts, catalytic diesel particulate traps, continuously regenerating traps, lean NOx traps or diesel oxidation catalysts operate or other treatment systems operate, the metal can scavenge the sulfur and form stable metal sulfates. This scavenging process then ties up the sulfur and protects the catalyst from sulfur deposition. Suitable exhaust temperatures in the vicinity of the LNT are those temperatures that enable the formation of some stable sulfates with the metal from the organometallic scavenger. Typical exhaust temperatures are below 650xc2x0 C., preferably below 600xc2x0 C. and more preferably below about 500xc2x0 C. For example, from about 200 to about 650xc2x0 C.
Suprisingly, when a compound according to the present invention is used in a fuel containing sulfur, especially low amounts of sulfur, the conversion efficiency of emissions control systems is maintained at a much higher rate than when the base fuel is used alone.